An epoxy resin composition is excellent in physical strength and adhesion and has been widely used as an adhesive, putty material, paint and coating material. Since the conventional epoxy resin composition uses a highly reactive amine compound as a hardener, it is a two-pack type characterized in that an epoxy resin and the hardener component are mixed together right before use of the two-pack type. However, since the two-pack epoxy resin composition requires such operations as measuring and mixing, it has poor workability. Further, due to complication of the operations, the two-pack epoxy resin composition also has problems such as a measuring error and inadequate mixing. In addition, the two-pack epoxy resin composition also has a problem that time in which it can be used is limited since a chemical reaction in the two-pack type is initiated by mixing.
Under the circumstances, a variety of studies on a one-pack epoxy resin composition have been made, and a number of techniques for a one-pack epoxy resin composition using a moisture hydrolyzable latent hardener, notably a ketimine compound and an oxazolidine compound, are known. In particular, from an industrial standpoint, a variety of techniques for a one-pack epoxy resin composition using a ketimine compound obtained from methyl isobutyl ketone as a carbonyl compound have been disclosed.
The ketimine compound and the oxazolidine compound are well-known as a latent hardener for an epoxy resin and an isocyanate-terminated urethane polymer. Hereinafter, a reaction mechanism of a composition comprising the ketimine compound or oxazolidine compound as a latent hardener and an epoxy resin will be described. Firstly, as a first reaction, the ketimine compound reacts with moisture in the air and is hydrolyzed so as to produce a primary amine compound having active hydrogen. The oxazolidine compound reacts with moisture in the air so as to produce a secondary amino alcohol. Thus, the first reaction is a process in which the latent hardener is hydrolyzed by moisture. Subsequently, as a second reaction, the produced amine compound having active hydrogen reacts with the epoxy resin, and by this mechanism, the epoxy resin composition is cured. Hence, the second reaction is a process in which the hydrolyzed latent hardener chemically reacts with the epoxy resin. That is, the reaction mechanism of the composition comprising the latent hardener and the epoxy resin is a two-step reaction comprising the reaction of the latent hardener with moisture and the reaction of the amine compound with the epoxy resin. In these two processes, the most important points with respect to the compositions comprising these latent hardeners and the epoxy resin are that (1) the quicker the hydrolysis of the latent hardener such as the ketimine compound proceeds, quicker-curability can be obtained and that (2) the higher the reactivity of the amine compound resulting from the hydrolysis, the more easily physical properties such as quick curability and high strength are obtained. However, when a ketimine compound which is hydrolyzed quickly is used, the ketimine compound is liable to be hydrolyzed during production or storage of the one-pack epoxy resin, so that it becomes difficult to obtain good storage stability. Hence, it was the limitation of the prior art that it could not help but relying on means using a ketimine compound obtained from an amine compound having high reactivity with the epoxy resin and having low hydrolyzability, in consideration of storage stability. Thus, since there is a dilemma that an improvement in quick curability causes impairment of storage stability, a technique of achieving practical quick curability and practical storage stability simultaneously in the composition comprising the ketimine compound and the epoxy resin is not yet found at all.
Meanwhile, recently, a technique of improving storage stability by use of a specific ketimine compound obtained from a carbonyl compound having steric hindrance is disclosed in WO98/31722. The specific ketimine compound has low hydrolyzability since water hardly makes contact with a site showing hydrolyzability due to its steric structure. Therefore, the ketimine compound has the conventional problem that although it can impart good storage stability, it fails to impart good curing properties such as quick curability. That is, when the specific ketimine compound is used, there arises a problem that curing of an epoxy resin composition proceeds slowly, so that initial adhesive strength and mechanical strength are slow to become in effect. It takes long-time curing to attain practicable physical properties, so that the ketimine compound is practically unsatisfactory. Consequently, even this technique is not a technique capable of achieving practical curing properties and practical storage stability simultaneously. That is, it is a technical means which is an extension of the prior art.
Thus, if a composition having excellent storage stability and excellent curing properties is found out of compositions comprising a ketimine compound or oxazolidine compound as a latent hardener and an epoxy resin, it becomes a fundamental technique for an adhesive, putty material, paint, coating material and potting material using them, so that usefulness of such a composition in industry is significantly improved.
Under the circumstances, an object of the present invention is to provide a one-pack moisture curable epoxy resin composition which can be cured at room temperature, shows balanced contradictory properties, i.e., has significantly excellent storage stability without impairing curability, and also has excellent deep curability.